The DieCAST Model



 

Model Description
Programmer's Guide

Results:

Model Description

The DieCAST (Dietrich Center for Air Sea Technology) model is a primitive equation, z-level ocean and lake model patterned after the Sandia Ocean Modeling System (SOMS). The model is hydrostatic, incompressible, rigid-lid, partially implicit, and fully conservative. The numerical differences from the SOMS model stem only from using an Arakawa A grid instead of the Arakawa C grid used by SOMS. The DieCAST model is simpler and requires less computation per time step than the SOMS model. It is also stable with substantially longer time step. Both DieCAST and SOMS use higher order treatment of the generally dominat terms in ocean dynamics, i.e. pressure gradient and Coriolis. This leads to accurate and robust (stable with realistically small dissipation) models.

Gulf of Mexico

The DieCAST model has been applied successfully to the Gulf of Mexico [shown above] with different model parameters and forcing functions. Figure 1, Figure 2, and Figure 3 show results from a 1/12 degree resolution model with 20 z-levels. The surface forcing and boundary conditions of this experiment come from:
  1. Annual wind stress from Hellerman and Rosenstein
  2. Annual temperature and salinity from Levitus
  3. Caribbean inflow data reported by Schmitz and Richardson adjusted to match the 25 Sverdrup Florida Strait flow estimated from calibrated phone cable data between Havana and Key West.
The model starts at rest with specified inflow conditions based on observations. Annual mean wind forcing from Hellerman and Rosenstein and restoring to Levitus climatology are applied at the surface.

North Atlantic Basin

We use a modified Arakawa "a" grid (semi-collocated) model that has recently been modified to greatly reduce numerical dispersion (Dietrich and Martin, Proceedings of ASCE Fourth International Estuarine and Coastal Modeling Conference, San Diego, CA, 26-28 October, 1995). Combined with its small explicit diffusion parameters, this produces a highly intertial model which admits many eddies. The results show a realistic Labrador Current leading to full GS separation at Cape Hatteras. This is seen in Figure 1 which compares model snapshot with observations, and in the model, animation

Model Setup

Region Modeled: 15 deg south to 65 deg north latitude
0 to 80 deg west longitude

This is the domain that Beckman, et al (JPO, vol 24, pp. 326-344) used in WOCE model studies.

Complete thermodynamics is included with surface restoring times from 180 days to annual mean Levitus T and S. The restoring follows application of long-term average surface fluxes (derived from previous restoring increments) each time step. Unfiltered real bottom topography is used. All resolved islands (Bahamas, etc.) are included. Annual mean Hellerman winds are applied. Resolved scales accomplish vertical mixing (no instant convective adjustment). The Gulf of Mexico and Western Caribbean are replaced by a narrow channel through Cuba near the model's western boundary (80 deg west) that is patterned after the Yucatan Strait. In other respects, the model is similar to the WOCE model.

We use a closed southern boundary with Newtonian damping to climatology in the adjacent region. We specify a 7.5 Sv east Greenland Current inflow through the Denmark Strait and allow the return flow to exit east of Iceland.

Resolution Parameters

Longitudinal Resolution: 1/2 deg longitude
Latitudinal Resolution: Variable such that DY-DX at all latitudes
Vertical Resolution: 10 layers with top layer 40 m thick, uniformly expanding vertical grid to a maximum depth of 5000 m.

The model runs 500 model days per cpu-day on a 12.5 megaflop Silicon Graphics workstation. This translates to over a century per cpu-day on a one-gigaflop supercomputer with 1/2 degree resolution.

Physical Parameters

Horizontal Eddy Viscosity and Diffusivity: 5 m-m/sec (constant)
Vertical Viscosity and Diffusivity: 1 cm-cm/sec plus contribution proportional to vertical velocity.
Total< 10 cm-cm/sec.
Bottom Drag Coefficient: 0.002

Other Applications

The model has also been applied to the South China Sea, the Labrador Current, the Strait of Sicily, the Great Lakes, East Australian Current, the North Atlantic Ocean, the Arctic Ocean, and the California Current.

For further information see the references, or contact Dr. David Dietrich or Avichal Mehra.

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